Primary open-angle glaucoma (POAG) is an ocular disease characterized by progressive optic nerve damage, visual field loss and in many cases, elevation of intraocular pressure (IOP). Therapeutically lowering IOP confers protection against progression of POAG, and is the only known treatment for POAG. But simply lowering IOP is not always enough, and glaucomatous progression and blindness are not uncommon in patients receiving IOP-lowering treatment for glaucoma. IOP is known to fluctuate over time, and large fluctuations of IOP in treated glaucomatous eyes have recently been associated with progression despite mean IOP levels that appear to be therapeutically adequate. Thus, IOP variability is an important parameter to assess. The current model for IOP variability is single-day diurnal IOP variability. Our knowledge of diurnal IOP behavior is based on many single-day 24-hour (or less) diurnal IOP assessments in normal and glaucomatous eyes. Based on these one-day-in-time measurements, it is assumed that this IOP rhythm is symmetric between fellow eyes of individuals and remains a constant rhythm from day to day. This proposal challenges that dogma. Based upon our own preliminary studies and a very few studies which suggest that diurnal IOP rhythms change in patients over time, we hypothesize that IOP variabilityis a chaotic event over time in both normal and glaucomatous eyes, and that knowing the diurnal IOP rhythm of a given eye on a given day does not permit prediction of the diurnal rhythm on any other day. Disproving the existence of a regular and sustained diurnal IOP rhythm would force a paradigm shift toward more long-term indices of IOP variability. By collecting diurnal IOP measurements at baseline and 1 day, 1week, 1 month, six months, and one year later, we can achieve our specific aims to characterize the short- (days), intermediate- (weeks to months), and long-term (year) diurnal variability of intraocular pressure in normal and glaucomatous eyes. We hypothesize that repeated diumal measurements of intraocular pressure in normal and glaucomatous eyes will demonstrate significant variability in the diurnal IOP rhythms between patients, between fellow eyes, and within patients and eyes over time.